It is well known to use microprocessors for controlling valves in order to achieve a desired pressure of a hydraulic fluid in a clutch in an automatic transmission. Present systems utilize heuristic rules that assume that opening a valve for a certain period of time is proportional, i.e., bears a linear relationship to, a desired pressure in a transmission clutch. Thus, once this linear relationship is programmed into a logical controller, the system is forced to assume that causing a certain valve behavior is properly calibrated to the resulting pressure within the clutch. However, even if the calibration is initially accurate, over time it becomes less so due to the wear of parts in the transmission, degradation of transmission fluid, inherent nonlinearities in the system's behavior, etc. Moreover, present systems are inherently prone to error while in operation due to electrical load variations, pressure pulsations, and other system non-linearities.
In short, present transmission control systems are not able to re-calibrate, or apply corrections to, their pre-programmed heuristic rules while in operation. That is, present transmission control systems may be characterized as “open loop” in that they provide input to a transmission but fail to take advantage of information that could be provided from the transmission system as output. Accordingly, there is a need for a transmission pressure control system that accounts for the non-linearity that exists in practically every such system. It would be further desirable in such a system to eliminate or significantly reduce steady state error.
Further, as is known, vehicle performance, and in particular transmission performance, changes over time as vehicle parts, such as transmission parts, age and become worn. However, because they are open-loop, present transmission control systems are unable to calibrate the pressure applied in a clutch in a transmission to the present age and wear of the transmission. Present open loop systems simply are not capable of accounting for the wear and age of transmission parts.
Therefore, it would be advantageous to have a transmission control system in a vehicle that applied an amount of pressure on a transmission clutch that was appropriate for the age and wear of the transmission. Such a transmission control system would have the benefits of improved fuel economy and better “drivability”, i.e., the transmission would shift more smoothly than it would with a conventional open loop control system. Further, such a transmission control system would have the benefit of enabling parts in a transmission to accumulate less wear as they age. Further, such a transmission control system would have the benefit of lowering the cost of parts in a transmission. This is because such parts would be required to have lower tolerances to fluctuations in pressure and other forces than today's transmission parts, thus lowering design and manufacturing costs.
Accordingly, there is a need for a transmission pressure control system that uses information about pressure states in the transmission to calibrate valve behavior with desired pressures.